Haemostasis is initiated by the formation of a complex between tissue factor (TF) being exposed to the circulating blood following an injury to the vessel wall, and FVIIa which is present in the circulation in an amount corresponding to about 1% of the total FVII protein mass. This complex is anchored to the TF-bearing cell and activates FX into FXa and FIX into FIXa on the cell surface. FXa activates prothrombin to thrombin, which activates FVIII, FV, FXI and FXIII. Furthermore, the limited amount of thrombin formed in this initial step of haemostasis also activates the platelets. Following the action of thrombin on the platelets these change shape and expose charged phospholipids on their surface. This activated platelet surface forms the template for the further FX activation and the full thrombin generation. The further FX activation on the activated platelet surface occurs via a FIXa-FVIIIa complex formed on the surface of the activated platelet, and FXa then converts prothrombin into thrombin while still on the surface. Thrombin then converts fibrinogen into fibrin which stabilizes the initial platelet plug. This process is compartmentalized, i.e., localized to the site of TF expression or exposure, thereby minimizing the risk of a systemic activation of the coagulation system. The fibrin forming the plug is furthermore stabilized by FXIII-catalyzed cross-linking of the fibrin fibers; this makes the clot insoluble.
FVIIa exists in plasma mainly as a single-chain zymogen, which is cleaved by FXa into its two-chain, activated form, FVIIa. Recombinant activated factor VIIa (rFVIIa) has been developed as a pro-haemostatic agent. The administration of rFVIIa offers a rapid and highly effective prohaemostatic response in haemophilic subjects with bleedings who cannot be treated with coagulation factor products due to antibody formation. Also bleeding subjects with factor VII deficiency or subjects having a normal coagulation system but experiencing excessive bleeding can be treated successfully with FVIIa. In these studies, no unfavourable side effects of rFVIIa (in particular the occurrence of thromboembolism) has been encountered.
Extra exogenously administered FVII a increases the formation of thrombin on the activated platelet surface. This occurs in haemophiliac subjects lacking FIX or FVIII and therefore missing the most potent pathway for full thrombin formation. Also in the presence of a lowered number of platelets or platelets with a defect function, extra FVIIa increases the thrombin formation.
Clot lysis is mediated by plasmin in vivo. Under natural conditions, plasminogen is convened to plasmin by tissue plasminogen activator (t-PA). Activation occurs on the fibrin surface, thus confining proteolytic activity to the appropriate site. After plasmin is set free into the circulation, it is rapidly combined with natural inhibitors. Inactivation of plasmin is the final and necessary step in the process of protecting against undesirable proteolysis. Alpha2-antiplasmin is the main physiologic plasmin inhibitor in mammalian plasma. Alpha2-antiplasmin is synthesized in the liver and is present in plasma at a concentration of about 1 μM. Human and murine alpha2-antiplasmin are serpins (serine proteinase inhibitors) with molecular weight (Mr) 65 to 70 kD, which inhibit plasmin in a very rapid reaction resulting in the formation of a stable inactive complex. The cDNA and deduced amino acid sequence, as well as the gene organization of both human and murine alpha2-antiplasmin have been elucidated (Holmes W E. et al. J Biol Chem 262:1659, 1987; Menoud P A. et al. J Clin Invest 97:2478, 1996).
It is well known that subjects who bleed excessively in association with surgery or major trauma and need blood transfusions develop more complications than those who do not experience any bleeding. However, also moderate bleedings requiring the administration of human blood or blood products (platelets, leukocytes, plasma-derived concentrates for the treatment of coagulation defects, etc.) may lead to complications associated with the risk of transferring human viruses (hepatitis, HIV, parvovirus, and other, by now unknown viruses). Extensive bleedings requiring massive blood transfusions may lead to the development of multiple organ failure including impaired lung and kidney function. Once a subject has developed these serious complications a cascade of events involving a number of cytokines and inflammatory reactions is started making any treatment extremely difficult and unfortunately often unsuccessful. Therefore a major goal in surgery as well as in the treatment of major tissue damage is to avoid or minimise the bleeding. To avoid or minimise such bleeding it is of importance to ensure the formation of stable and solid haemostatic plugs that are not easily dissolved by fibrinolytic enzymes. Furthermore, it is of importance to ensure quick and effective formation of such plugs or clots.
Today, subjects experiencing bleeding episodes, including trauma victims and subjects bleeding in association with surgery, are often treated with several injections or infusions of FVIIa since the short half-life of FVIIa (2.5 hours) may require more than one administration to maintain a certain level of haemostatic ability. A faster arrest of bleedings would be an important benefit to such subjects. So would a reduction in the number of administrations needed to stop bleeding and maintain haemostasis.
European Patent No. 225.160 (Novo Nordisk) concerns compositions of FVIIa and methods for the treatment of bleeding disorders not caused by clotting factor defects or clotting factor inhibitors.
European Patent No. 82.182 (Baxter Travenol Lab.) concerns a composition of factor VIIa for use in counteracting deficiencies of blood clotting factors or the effects of inhibitors to blood clotting factors in a subject.
International Patent Publication No. WO 93/06855 (Novo Nordisk) concerns the topical application of FVIIa.
There is still a need in the art for improved treatment of subjects experiencing bleeding episodes, including subjects where the bleeding episodes are due to surgery, trauma, or other forms of tissue damage; induced coagulophathy, including coagulopathy in multi-transfused subjects; congenital or acquired coagulation or bleeding disorders, including diminished liver function (“liver disease”); defective platelet function or decreased platelet number; lacking or abnormal essential clotting “compounds” (e.g., platelets or von Willebrand factor protein); increased fibrinolysis; anticoagulant therapy or thrombolytic therapy; or stem cell transplantation.
There remains a need in the art for an improved, reliable and widely applicable method of enhancing coagulation, enhancing or ensuring formation of stable haemostatic plugs, or enhancing convenience for the treated subject, or achieving full haemostasis in subjects, in particular in subjects having an impaired thrombin generation. There is also a need for methods wherein the time to bleeding arrest is shortened.